Vibration assisted processing of viscous thermoplastics

ABSTRACT

Cure and consolidation of a viscous thermoplastic resin, especially one that softens and melts at a high temperature, is improved by assisting autoclave pressurized momentum transport for compaction of the composite using high frequency, low amplitude acoustic vibration within the resin. The vibration induces shear thinning in the resin which promotes resin flow, achieves filling or collapse of voids, and assists consolidation of the composite preform.

REFERENCE TO RELATED APPLICATION

The present application is a divisional application based upon U.S.patent application Ser. No. 08/762,235, filed Dec. 9, 1996, now U.S.Pat. No. 6,592,799. The present application is also related to U.S.patent application Ser. No. 08/912,157, filed Aug. 15, 1997, now U.S.Pat. No. 6,033,203, which also was a divisional application based uponSer. No. 08/762,235.

TECHNICAL FIELD

The present invention relates to a method and associated tooling toassist flow, consolidation, and cure of a fiber-reinforced, viscousthermoplastic resin composite by inputting vibration energy at highfrequency and low amplitude displacement to supplement conventionalautoclave heating, especially for high melting, non-Newtonian,pseudoplastic resins.

BACKGROUND OF THE INVENTION

Aerospace thermoplastic composites are relatively difficult to processbecause the resins contain significant amounts of solvent and cure atrelatively high temperatures often with a limited range of temperaturebetween the boiling point of the solvent, melting point of the resin,and curing temperature of the resin. We call this temperature range theprocessing window with conventional autoclave processing where theprepreg laminate is enclosed within vacuum bags and heated within apressurized oven, it is often difficult to obtain substantially fullyconsolidated products. Operating in the narrow processing window isdifficult, but doing so is essential to evaporate the solvent, to meltthe resin so that plies in the laminate will consolidate and flow, andto cure the resin by its chain extension condensation reaction.Augmenting the processing with ultrasonic vibration to supplement theconventional practice of pressing the melted material for momentumtransport (“flow”) should improve the products while reducing the curecycle. Therefore, the process of the present invention saves time andreduces waste or rework. Since the resins cost over $100 per pound andthe manufacturing process is relatively slow and labor intensive, thepresent process promises a significant economic benefit.

In U.S. Pat. No. 4,288,398, Lemelson described alternative methods forcontrolling the internal structure of molded or extruded plastics ormetals. Lemelson suggested using ultrasound alone or in combination withother forms of energy to orient the grain or crystalline structure.Lemelson introduced ultrasound to the melted material during itsconsolidation to control the internal structure. The process of thepresent invention uses ultrasound to assist momentum transport afterheating the resin to its softening or melting temperature and during thepressure application for resin flow phase of its consolidation.

Resins that cure to high operating temperature thermoplastic compositesgenerally require high processing temperatures. For a resin-fibercomposite system capable of operating at 425° F. or higher, the resinmust have a glass transition temperature (T_(g)) of 525° F. afterequilibration with the operating environment and an “as processed” T_(g)approaching 600° F. A resin with a high T_(g) will also have a highmelting (T_(m)) or softening (T_(s)) temperature. The temperaturedifferential between the T_(g) and the T_(s) is established by themolecular weight distribution and usually is on the order of 200° F. Inaddition, the viscosity of such a high melting resin above the melt orsoftening temperature will likely be greater than 10⁶ Pa·sec. Therefore,consolidating acceptable quality laminates using these resins requireshigh pressures and temperatures.

To date, only small parts that will fit within the platens of a presscould be fabricated using extremely high T_(g) resins. Processing inthis manner requires matched tooling and has been limited to high valueparts such as engine components.

Attempts to consolidate large planform area parts (such as exterior skinpanels for composite aircraft) in an autoclave have been unsuccessful.The laminates exhibited extensive porosity and suffered frommicrocracking because of the volatiles and by-product gases generatedduring the condensation reaction of the resin when it cured. Highviscosity of these high melting resins inhibited momentum transport andresin flow during the pressurized portion of the autoclave cycle. Whileprocessing might be possible at even higher temperatures and pressures,conventional autoclaves are not designed for the increased pressures.Replacing conventional autoclaves to allow higher pressure operation istoo expensive to justify using the high melting resins available todayfor today's applications.

SUMMARY OF THE INVENTION

In the present invention, piezoelectric transducers apply vibration athigh frequency (in excess of 10⁵ Hz) and low displacement (i.e., ≦1 μm)to a prepregged part on a layup mandrel to advance the consolidation ofcomposites containing high viscosity resins that have high meltingtemperatures, high glass transition temperatures, and exhibitpseudoplastic rheology. The resin flow is increased by subjecting theparts to be processed to high frequency vibration (high shear rates),which usually exceeds 10⁶ Hz. The vibration causes the melted resin toflow into voids in the part and incrementally heats the resin to assistin its consolidation. To avoid distortion of the individual fibers,yarns, or plies the displacement must be limited. If individual fiberstranslate relative to each other (i.e., move independently), then thedisplacement should not exceed 5-10 μm, which is comparable to the fiberdiameter. If the yarns or individual plies vibrate together, then thedisplacement can be increased to 0.005-0.01 inches. The viscosity versusshearing rate behavior will be pseudoplastic i.e. shear thinning, toprovide the desired effect on the viscous resin. At these sufficientlyhigh shear rates (one reference states above 10⁶ Hz), the viscosity ofthe pseudoplastic resin will revert to Newtonian behavior which willfacilitate flow of the resin at pressures that are achievable withconventional autoclaves.

Processing of a laminate containing a high viscosity resin includes (1)laying up prepreg plies by hand or with fiber placement machines in adesired pattern on a layup mandrel, (2) applying a suitable vacuum bagaround the plies, and (3) placing the bag in either a specially equippedoven or an autoclave for consolidation. The surrounding atmosphere ispressurized to apply pressure to the vacuum bagged part. The baggingmight include a diaphragm chamber that can be pressurized or the entireautoclave can be pressurized, or both approaches can be used. In thepresent invention, the oven or autoclave is equipped to provide highfrequency, low displacement vibration to the part during the heating andpressure application phase of the consolidation cycle to promote resinflow by converting the rheology from pseudoplastic to Newtonian.

Our approach involves installing piezoelectric transducers in the layupmandrel and providing electrical power through an appropriate connectionafter positioning the tool in a conventional oven or autoclave. Thepropagation of acoustic waves (i.e., the pressure wave imparted by thetransducer at ultrasonic frequencies) through a the part involvesperiodic fluctuations in pressure and displacement. At a high powerdensity, the pressure amplitude can reach or exceed 1000 psi atdisplacements less than 1 μm (10⁻⁶ m). The ultrasonic vibration aidsmass transport of volatiles out of the resin by increasing theprobability of nucleation

Ultrasonic vibration may also aid in the compaction of dry thermoplasticprepreg tape that is being laid with a hot-head automatic tape layer.The vibration may aid in the local compaction of the material under thetape head when the material is laid down to reduce the pressurerequirement during the laying process. A reduced pressure may allow theuse of a conformal rubber application head rather than a conventionalrigid head.

We can also introduce the ultrasonic energy into a part by placing acover panel or blanket containing the piezoelectric transducers over thepart surface to achieve the necessary conduction path.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an ultrasonic vibration table of the present inventionpositioned in an autoclave for receiving a layup mandrel.

FIG. 2 shows a side elevation of a typical vibration table of the typeshown in FIG. 1.

FIG. 3 shows a transducer pad positioned over a layup mandrel.

FIG. 4 shows a cross-section of the transducer pad on the layup mandrel.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention relates to a method for processing advancedcomposite thermoplastic resins, like duPont's K3B, to produce fullyconsolidated (essentially zero porosity) parts using ultrasonicvibration at high frequency and low amplitude to promote flow. Theresins of interest exhibit non-Newtonian Theological behavior. Insteadof having a generally linear relationship between shear stress and therate of shear, advanced composite resins typically are non-Newtonianpseudoplastic fluids that are characterized by a decrease in viscositywith an increase in the shear rate. As pseudoplastics, the increase ofshear rate, therefore, promotes resin flow. In the present invention, wesupplement the Theological properties achievable through the dynamiccompression stress and elevated temperatures that we can obtain inconventional autoclave processing by imparting vibration to the part.

Some advanced composite resins require higher processing pressures thanautoclaves can achieve, so the ultrasonic vibration augmentation of thepresent invention is crucial if these resins are to be processed to zeroporosity structural aerospace parts. The ultrasonic vibration translatesto a dynamic pressure enhancement to the static pressure of theautoclave.

The present invention is useful both for the curing and consolidation ofresins and for their injection into resin transfer molding (RTM) tools.Enhancing flow with the increased shear rate allows the resin's solventto escape and the resin to flow around the reinforcing fiber to achievefull saturation. Enhanced flow allows processing in a narrow processingwindow in which the solvent boils, the resin melts, and the melted resincures. The resin must complete flowing before cure progresses to anysignificant extent because the cure causes the resin to chain-extend tomolecules of large molecular weight that have poor flow characteristics.

FIGS. 1 and 2 illustrates one preferred embodiment of how we introducehigh frequency, low amplitude ultrasonic vibration to prepregs 10. Theresin prepregs 10 are laid up on a layup mandrel 12 (FIG. 2) that restson hard point attachment feet 14 over a pair of metal plates 16 & 18sandwiching piezoelectric transducers 20. The plates typically are Invaror another metal having a coefficient of thermal expansion comparable tothe resin composite. The transducers convert electrical energy toultrasonic vibration. Suitable transducers should be functional at theelevated autoclave processing temperatures and pressures to input to thepart vibrations of at least 10⁵ Hz (and probably at least 10⁶ Hz) with adisplacement amplitude of no more than about 10⁻⁶ m. The frequencyshould exceed the frequency for the resin being processed at whichNewtonian Theological behavior appears in the pseudoplastic fluid stateof the resin for its melting and curing.

The metal plates 16 & 18 and piezoelectrical transducers 20 form a“shaker table.” The shaker table sits on a shock isolation mounting 22in the autoclave 24.

FIGS. 3 and 4 shows an alternate embodiment for imparting the ultrasonicvibrational energy as an acoustic wave to the resin. Here, a transducerpad or cover panel 30 overlies the resin on the layup mandrel 12. Abreather 26, release film 27, and vacuum bag 28 (FIG. 4) separate thetransducer pad 30 from the part 10. The transducer pad is shaped tocorrespond to the configuration of the completed part and acts as a caulplate for the autoclave pressure.

For RTM processing, the transducers act on the rigid tool surfaces.

Power requirements are difficult to calculate because damping effectsare difficult to model. Similarly, the correlation between power densityor intensity as a function of displacement and frequency are notcompletely understood. Nevertheless, at 10⁵ Hz and 10⁻⁶ m displacement,a typical power requirement is on the order of 15,000 W/m². Accordingly,large parts will require significant energy.

Cure and consolidation of a viscous thermoplastic resin, especially onethat softens and melts at a high temperature, is improved by assistingautoclave pressurized momentum transport for compaction of the compositeusing high frequency, low amplitude acoustic vibration within the resin.The vibration induces shear thinning in the resin which promotes resinflow, achieves filling or collapse of voids, and assists consolidationof the composite preform.

While we have described preferred embodiments, those skilled in the artwill readily recognize alterations, variations, and modifications whichmight be made without departing from the inventive concept. Therefore,interpret the claims liberally with the support of the full range ofequivalents known to those of ordinary skill based upon thisdescription. The examples are given to illustrate the invention and notintended to limit it. Accordingly, limit the claims only as necessary inview of the pertinent prior art.

We claim:
 1. A method for curing a viscous thermoplastic resin preformhaving significant residual volatiles, comprising the steps of: a)preparing a fiber-reinforced resin perform; b) enclosing the preform ina pressure zone; c) applying a suction to reduce the pressure below oneambient atmosphere in the pressure zone to extract residual volatilesand condensation by-products during curing; d) compacting the enclosedpreform; e) heating the preform in an autoclave to a curing temperatureof the resin while inputting acoustic vibration to the preform to assistmomentum transport of the resin for adequate flow to fill voids and toassist consolidation as the resin cures at the curing temperature;wherein the acoustic vibration is input into the preform throughtransducers integrated into a cover panel that overlies the preform on alayup mandrel, the cover panel being contoured to correspond with thecurvature of the preform.